The invention relates to a fuel injector for injecting fuel provided at a high pressure into the combustion space of an internal-combustion engine.
Such a known fuel injector comprises an injector housing and a nozzle needle which has a nozzle needle shaft, which is longitudinally displaceably disposed in a first guide bore constructed in the injector housing, and a nozzle needle point interacting in the sense of an opening and closing of a valve opening cross-section with a valve seat constructed in the forward end of the injector housing. For supplying highly pressurized fuel to be injected, a high-pressure duct is provided. On the face side of the first guide bore, a nozzle antechamber is disposed in front of the nozzle needle shaft, which antechamber is acted upon at a high pressure by the fuel to be injected which is supplied by way of the high-pressure duct. A control space acted upon by highly pressurized fuel is coupled with the nozzle needle, which control space can be relieved from pressure by a control valve in the sense of an opening of the nozzle needle. At the rearward side of the first guide bore, a space is arranged which receives fuel flowing from the nozzle antechamber by way of the first guide bore.
Such a fuel injector has the disadvantage that considerable leakage occurs between the nozzle antechamber and the space arranged on the rearward side of the first guide bore as well as between the control space and this space, which leakage may be in the range of up to 20 or 30% of the maximal injection quantity.
It is an object of the invention to construct a fuel injector of the initially mentioned type such that this leakage is avoided.
This object is achieved by way of a fuel injector having the space arranged on the rearward side of the first guide bore is a high-pressure space acted upon by a highly pressurized fuel.
Advantageous further developments of the fuel injector according to the invention are characterized in the preferred embodiments.
The fuel injector according to the invention is provided for injecting highly pressurized fuel into the combustion space of an internal-combustion engine. The fuel injector comprises an injector housing and a nozzle needle which has a nozzle needle shaft, which is longitudinally displaceably disposed in a first guide bore constructed in the injector housing, and a nozzle needle point interacting in the sense of an opening and closing of a valve opening cross-section with a valve seat constructed in the forward end of the injector housing. For supplying highly pressurized fuel to be injected, a high-pressure duct is used. On the face side of the first guide bore, a nozzle antechamber is disposed in front of the nozzle needle shaft, which antechamber is acted upon at a high pressure by the fuel to be injected which is supplied by the high-pressure duct. A control space acted upon by highly pressurized fuel is coupled with the nozzle needle, which control space can be relieved from pressure by a control valve in the sense of an opening of the nozzle needle. At the rearward side of the first guide bore, a space is arranged which receives fuel flowing from the nozzle antechamber by way of the first guide bore or from the control space. According to the invention, the space arranged on the rearward side of the first guide bore is a high-pressure space acted upon by a highly pressurized fuel.
The significant advantage of the fuel injector according to the invention is the fact that no space which is at a low pressure level is situated on the rearward side of the first guide bore guiding the nozzle needle, so that no leakage can occur by way of this space.
According to an aspect of the invention, the high-pressure space constructed on the rearward side of the first guide bore is formed by the control space. This results in the advantage that, as a result of the pressure existing in the control space, a flowing over of fuel by way of the first guide bore is not possible. Another advantage is the fact that, because of the direct action upon the nozzle needle by the pressure situated in the control space, a very rapid response behavior of the fuel injector is achieved. Since there is no low-pressure space adjoining the control space, a leakage from the control space cannot take place.
According to a further development of the invention, the control space forming the rearward-side high-pressure space contains a restoring spring acting upon the nozzle needle in the closing direction.
The restoring spring is advantageously formed by a cup spring arrangement. Preferably, it is provided that the restoring spring is supported on one end by a first abutment provided on the rearward side of the nozzle needle shaft and is supported on the other end by a second abutment constructed on the rearward side of the control space.
According to an advantageous embodiment of the invention, the control space forming the rearward-side high-pressure space is formed by a bore extending in the longitudinal direction of the injector housing and, on its rearward side, is bounded by a valve body of the control valve inserted into this bore.
The control space forming the rearward-side high-pressure space is preferably connected by way of a throttle duct with the high-pressure duct carrying the fuel to be injected.
According to a second aspect of the invention, the rearward high-pressure space is formed by a spring space containing the restoring spring acting upon the nozzle needle in the closing direction. In this respect, it is advantageous that the restoring spring can be optimally dimensioned, while simultaneously the control space can be constructed to be very small, which is advantageous for the response behavior of the fuel injector.
The spring space is preferably connected by way of a fluidic connection with the high-pressure duct carrying the fuel to be injected.
According to a particularly advantageous embodiment of this variant of the fuel injector according to the invention, a second guide bore is constructed at the rearward side of the spring space forming the high-pressure space, which second guide bore extends coaxially to the first guide bore carrying the nozzle needle shaft and in which a guiding piston is displaceably in the longitudinal direction disposed, which guiding piston is coupled by way of a needle stilt with the nozzle needle, the guiding piston bounding the spring space on its rearward side.
The control space is preferably constructed on the rearward side of the guiding piston, in which case the fuel which is present at a high pressure in the spring space and the restoring spring act upon the nozzle needle shaft in the sense of a closing of the nozzle needle and, when the control space is relieved from pressure, the nozzle needle is relieved by the control valve by the guiding piston in the sense of an opening.
The first guiding bore guiding the nozzle needle shaft preferably has a diameter D1. The spring space is formed by a third bore coaxial to the first guide bore, the diameter D2 of the third bore being larger than the diameter D1 of the first guide bore. The control space is formed by the second guide bore with a diameter D1xe2x80x2 which is coaxial to the first guide bore and the spring space.
According to a preferred embodiment, it is provided that the diameters D1, D1xe2x80x2 and D2 are mutually coordinated such that the needle stilt during the opening as well as during the closing of the nozzle needle is only stressed with respect to tension. As a result, a buckling or a one-sided contact of the nozzle needle stilt, which could result in a jamming, will be avoided.
According to a preferred embodiment, the first guide bore and the second guide bore have the same diameter D1. The resulting advantage is a simplification during the manufacturing of the fuel injector.
According to a preferred embodiment, the restoring spring is supported on one end by a first abutment provided on the rearward side of the nozzle needle shaft and is supported on the other end by a second abutment constructed on the rearward side of the spring space.
The control space preferably has a significantly smaller volume than the spring space.
According to an advantageous further development of the fuel injector according to the invention, it is provided that the injector housing contains on the rearward-side end an individual storage device for supplying highly pressurized fuel, which individual storage device is connected with the high-pressure duct carrying the fuel to be injected. Such an individual storage device can be implemented particularly in the case of the fuel injector according to the first embodiment of the invention, in which the rearward-side high-pressure space is formed by the control space because a significant amount of length is saved in this embodiment and can be utilized for the individual storage device.